Elimination of agglomeration by freezing of lignite or other moisture-containing or wetted aggregates during shipment



United States Patent 3,243,889 ELIMINATION OF AGGLQMERATION BY FREEZ- ING 0F LIGNITE 0R G'IHER MOISTURE-CON- TAINING GR WETTED AGGREGATES DURING SHIPMENT Robert C. Ellman, East Grand Forks, Minn., and John W. Belter, Grand Forks, N. Daln, assignors to the United States of America as represented by the Secretary of the Interior N0 Drawing. Filed Apr. 23, 1963, Ser. No. 275,171 6 Claims. (Cl. 349) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention is concerned with a method for prevention of agglomeration by freezing of amass of moisturecontaining particulate material, particularly lignite.

Lignite as mined'has found increasing use as an industrial fuel in recent years, due largely to economic factors. Since most of the mining operations are stripping operations, the cost per ton mined is low. However, the low initial cost is frequently largely offset by the tendency of the lignite to agglomerate during winter shipment in open-top steel rail cars Individual particles agglomerate by freezing into large lumps at the top and exterior surfaces of the car and freeze to 'car walls and bottom. These :agglomera'tes are difiicult to remove and also impede unloading of the usually unfrozen center of the car. As a result, extensive hand labor is required, thus increasing the time and cost of unloading.

Lignite as mined is saturated and contains 35 to 40% moisture in capillaries or pores. Upon exposure, this moisture begins evaporating and is released as vapor. Initially the evaporation rate is high, decreasing with progressive evaporation to a much slower rate. The moisture involved in this initial high rate release is in the order of 2% by weight of the lignite and is the principle cause of the agglomeration by freezing. Since the lignite is mined and immediately loaded into a rail car, it is at ground temperature or approximately 45 F. In the car, which is exposed to air temperatures which may be as low as 30 F., the moisture vaporized from the lignite contacts the colder walls of the car to form frost crystals. As the exterior particles of lignite in the car cool below freezing, the plane of frost formation moves progressively from the walls into the car. The frost crystals effectively cement individual lignite particles together to form agglomerates. Moisture released from the unfrozen particles in the center of the car contributes to this process by migration, and any spontaneous heating which drives additional moisture from the particles increases the 'degree of agglomeration. Other variables which influence this process include temperature con ditions during transit and time of transit. process will hereinafter in the specification and claims be referred to as agglomeration by freezing.

The use of mechanical shakers and other devices are but partially effective in overcoming the effects of agglomeration and unloading is virtually a hand operation. The use of sodium chloride and calcium chloride to prevent or reduce agglomeration by freezing, though employed in the case of surface-wetted bituminous coals, has not been successful in the lignite industry. Further, problems of corrosion and contamination are associated with the use of these freeze-proofing agents. The treatment of lignite with oil for freeze-proofing has also been tried but has been only partially successful.

It is accordingly an object of the present invention to prevent agglomeration of lignite by freezing by means of This overall a method which is simple, economical and does not adversely affect the value of the lignite as a fuel.

It has nowbeen found that theseobjectives may be accomplished by adding to the natural lignite an amount of predried lignite sufiicient to prevent agglomeration. The beneficial effect of the predried lignite is believed due to removal of vaporized moisture from void spaces and particle surfaces during transit by the hygroscopic predri'ed material.

Lignite, after mining, is transported to a tipple where it is crushed, screened to desired sizes and loaded into railroad cars. For powerplant use, lignite is generally crushed to such size that it will pass through a size screen opening of about 2 inches square. The crushed lignite will also contain variable proportions of fines which pass through a size screen opening of about /2 inch to inch square.

These fines, or a portion of them, when predried, have been found to be of particular-value in prevention of agglomeration of lignite. The fines may be dried in any conventional manner such as in an oven or by means of hot gases. A high degree of drying increases the ability of the fines to reabsorb moisture; however, the lower the moisture content of the dried fines, the greater is the cost of drying and the susceptibility to spontaneous combustion. Therefore, the optimum moisture content of the predried lignite is usually from about 5 percent to about 25 percent.

Cooling of-the predried lignite prior to addition to the natural (non-dried) lignite usually gives best results, although addition of predried fines at the drier discharge temperature ofl F. has been successful. Removal of dust (particles less than mesh-US. Standard Sieve Series) is also usually desirable. Both cooling and dust removal may be accomplished by sweeping with air. The dried lignite may also be sprayed with oil to further reduce dust. V V

The optimum ratio of predried lignite to natural lignite will vary, depending upon the characteristics of each. The determining factor will be the amount of moisture that must be absorbed compared to the capacity of the predried lignite to absorb it. Proportions of predried fines of about 10 percent to about 20 percent have generally been found to give good results.

The p'redriedlighite may be added to the naturallignite as the latter is loaded into railroad cars, in such a man- 'ner 'as fo'iesult in 'a relatively even distribution throughout the natural lignite. However, superior results are usually achieved by adding the predried lignite in a manner which concentrates it at the sides and bottom of the railroad car, since this is where agglomeration takes place. This may be accomplished by first charging the dried fines to the center of the car. Then, as the natural lignite only :is loaded at the center of the car, the dried fines are added simultaneously to the sides of the car to mix with the natural lignite as the car is filled.

In addition to preventing agglomeration, the predrying step serves to upgrade the dried portion of the lignite thus increasing the over-all heating value of the shipped lignite. This is particularly advantageous in the case of predried fines since the fines contain the highest percentage of surface moisture and are, therefore, most in need of upgradmg.

Examples Because of the many variables involved in agglomeration by freezing, data collected from carload commercial shipment are difiicult to analyze, and it is impossible to control the variables involved. A laboratory test method was therefore developed in which control of the variables could be achieved. The laboratory study was carried out in a test chamber in which the temperature could be reduced to minus 20 F. rapidly and uniformly. A total of 18 samples of l-cu. ft. volume each were placed in the chamber and compared in each test. Each sample was placed in a container, the walls of which are removed after exposure to the test conditions of temperature and time duration. The agglomerated sample is a cylindrical shape 1 foot high and of l-cu. ft. volume. Of particular importance in the test procedure and apparatus was the achievement of uniformity of cooling of the samples. Such factors as size of the unfrozen lignite particles, uniformity of application of any treating agent, and method of packing sample into containers, all had influence on the duplication of degree of agglomeration between samples. The measurement of the degree of agglomeration or the strength of the bonds which cement the particles to each other was developed in the laboratory study. Essentially, the test method consists of imbedding a balloon in a sample and recording the air pressure necessary to inflate the balloon and break open or rupture the sample after it had been frozen and agglomeration had occurred. The pressure required to rupture each sample is a relative measure of the strength of the bonds holding the agglomerated particles together. For example, a pressure of 75 pounds per square inch gage and above indicates severely agglomerated samples; 45 to 60 indicates a moderate degree of agglomeration; and 28 to 32 is virtually unagglomerated though sutficiently so that sample container shape is retained. In this test work, variables were limited and as closely controlled as was possible in order to achieve maximum reproducibility. Particle size of the lignite tested was closely controlled. The results of these tests demonstrated the superiority of the addition of dried lignite as a freezeproofing agent.

Dried lignite was prepared from closely sized fines which passed through a inch square opening and were retained on a screen with openings of /8 inch square. This dried material was mixed with natural lignite to give a high degree of homogeneity to the sample. The particle size of the natural lignite was such as to pass a /2 inch square opening screen and be retained on a inch square opening screen. Results of one test in which the quantity and moisture content of the dried fines was studied are given in Table I.

['ABLE I.-RUPTURE 'PRESSURES OF AGGLOMERATED LIGNITE OF 35.2 PERCENT MOISTURE CONTENT, P.S.I.G.

' Percent of dried fines added Moisture content of dried fines, percent 2i eration which did occur was reduced so as to practically eliminate unloading ditficulties.

The effectiveness of the dried lignite in preventing agglomeration is believed due to two principal reasons: dried lignite is hygroscopic and absorbs moisture from the atmosphere and, secondly, the dried lignite does not contribute to the moisture-releasing mechanism and, therefore, acts as a diluent. Effectiveness of the fines is believed due to improved distribution and dispersion of the dried material in the natural lignite. An additional advantage of the use of the fines results from the fact that thermal drying of lignite produces fines due to degradation of larger particles and, therefore, the use of fines results in economical operation.

Though the method of the invention has been described in connection with lignite, it may also be employed to prevent agglomeration by freezing of other masses of moisture-containing particulate material such as other types of coal, iron ore and other minerals.

What is claimed is:

1. In a method for retaining in a container a mass of moisture-containing particulate material under conditions wherein said material normally agglomerates by freezing in the container, said material selected from the group consisting of coal and ore, the improvement comprising admixing with said moisture-containing particulate material an amount of said particulate material in a predried hygroscopic state so that said moisture-containing material and hygroscopic material are in a commingled state in the container after being added to the container, the hygroscopic material being added in an amount sufiicient to prevent said moisture-containing particulate material in said container from agglomerating by freezing.

2. The method of claim 1 wherein said container is arail car.

3. The method of claim 2 wherein the predried, hygroscopic material is concentrated at the sides and top and bottom of the car.

4. The method of claim 1 wherein the material is lignite.

5..The method of claims 4 wherein said predried, hygroscopic material is lignite fines.

6. The method of claim 5 wherein said predried hygroscopic lignite fines contain about 5% to about 25% moisture and are employed in an amount of about 15% to about 25% by weight of the material in the container.

References Cited by the Examiner UNITED STATES PATENTS 703,393 7/1902 Edson 34- 9 1,932,830 10/1933 Puening 34-102 1,979,124 10/1934 Tival 34-9 2,119,615 6/1938 Wendeborn 34-9 2,185,129 12/1939 Maus 34-9 2,290,068 7/ 1942 Peterson 34-102 X 2,534,051 12/1950 Nelson 349 2,610,115 9/1952 Lykken 34-9 WHLIAM F. ODEA, Primary Examiner.

NORMAN YUDKOFF, Examiner.

F. E. DRUMMOND, D. A. T AMBURRO, Assistant Examiners, 

1. IN A METHOD FOR RETAINING IN A CONTAINER A MASS OF MOISTURE-CONTAINING PARTICULATE MATERIAL UNDER CONDITIONS WHEREIN SAID MATERIAL NORMALLY AGGLOMERATES BY FREEZING IN THE CONTAINER, SAID MATERIAL SELECTED FROM THE GROUP CONSISTING OF COAL AND ORE, THE IMPROVEMENT COMPRISING ADMIXING WITH SAID MOISTURE-CONTAINING PARTICULATE MATERIAL AN AMOUNT OF SAID PARTICULATE MATERIAL IN A PREDRIED HYGROSCOPIC STATE SO THAT SAID MOISTURE-CONTAINING MATERIAL AND HYGROSOCPIC MATERIAL ARE IN A COMMINGLED STATE IN THE CONTAINER AFTER BEIND ADDED TO THE CONTAINER, THE HYGROSCOPIC MATERIAL BEING ADDED IN AN AMOUNT SUFFICIENT TO PREVENT SAID MOISTURE-CONTAINING PARTICULATE MATERIAL IN SAID CONTAINER FROM AGGLOMERATING BY FREEZING. 